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Undrained Strength

Undrained Strength. CIVE 640. So far…. 3 test types: 1. CD 2. CU 3. UU. 3 steps for each step 1. soil specimen obtained 2. consolidation phase (und. or dr.) 3. shearing phase (end = failure). CU – Weald clay. Board – Figure 28.6. 1. undrained = no vol change

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Undrained Strength

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  1. Undrained Strength CIVE 640

  2. So far… 3 test types: 1. CD 2. CU 3. UU 3 steps for each step 1. soil specimen obtained 2. consolidation phase (und. or dr.) 3. shearing phase (end = failure)

  3. CU – Weald clay Board – Figure 28.6 1. undrained = no vol change 2. oc clay is dilative 3. no peak is observed 4. for dilative clay, ue during shear = (-). Thus, the effective stress inc during shear 5. nc clay is contractive (ue during shear = (+)

  4. CU – Weald clay – eff stress paths Board – Figure 28.8 a 1. ESP is curved (backwards) for NC clay 2. As OCR approaches 1, backwards curvature increases 3. OCR may be such that ue @ failure = 0 4. A highlights the difference between ESP and TSP

  5. ESP The ESP = fn (OCR and p’ - e conditions before shear, i.e., p’o and eo )  fn (loading type)

  6. Drained vs. Undrained Board – Figure 28.8 a 1. qf - p’f relationship (envelope in MIT space) is independent on drainage conditions 2. Indeed, there is a qf - p’f - e relationship for a clay with a given OCR, independent on drainage conditions

  7. Undrained Shear Strength Undrained Strength qf = fn (effective stress) qf = fn (p’o , Af, f’, and c’) and is also a fn of drainage conditions but  fn (loading type) fn (OCR)

  8. Undrained Shear Strength Board – Table 28.1

  9. “f = 0” Consider a clay with p’o = 16 psi 3 UU tests (no additional cons. allowed in step 2 ): 1. po = s3 = 10 psi 2. po = s3 = 30 psi 3. po = s3 = 100 psi qf for all = 5 psi Why? Confinement change = u change. Since the effective stress = ct during this step, the strength (= fn eff stress) is constant.

  10. “f = 0” continued Board – Table 28.2 If qf is plotted vs. po A family of straight lines appears on the MIT space qf = fn (p’o)

  11. CSSM - Undrained Undrained No vol change = No e change Contractive (B) u = + during shear s’ decreases Dilative (A) u = - during shear s’ increases Santamarina et al 2002

  12. Cavitation • Occurs when –u < ~15 psi (1 atm) • May occur during shear, leading to “drained-type” behavior. • Peak strength becomes a function of the total confining stress po • More pronounced in sands

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